1. Field of the Invention
The present invention relates to measuring stress in materials. In particular, the present invention is a method for measuring residual stress in a variety of materials including metals using laser interferometry. The United States Government has rights in this invention pursuant to Contract No. DE-AC09-89SR18035 between the U.S. Department of Energy and Westinghouse Savannah River Company.
2. Discussion of Background
Materials used in construction that are made of metals and metal alloys are prone to a phenomenon known as stress corrosion cracking. In this phenomenon, cracks appear in areas of tensile stress, such as welded joints, as a result of the migration of chloride atoms to grain boundaries in the material. Tensile stresses are induced in manufacturing operations such as bending, heat treating, grinding and welding. The presence of stress in the material is one factor in the cause of stress corrosion cracking, but other factors are also important. Two additional factors are the specific metallurgical makeup of the material and the environment of use.
Residual stresses add to the load applied to a part used in construction. If the structural design is not sufficiently conservative, the part can fail from the combination of the load and material stresses. By having a method for accurately measuring the residual stresses in individual parts, a designer can have a better understanding of the total loads on them. With that understanding, the designer can predict failure with greater certainty and design to avoid failure or to relieve the stresses.
Stress measurement at the surface of a metal object is especially important because most failures begin them. Surface stress will result in crocks that propagate more rapidly, that tend to pull the material apart. Welding in particular imparts surface stresses because it imposes sharp temperature gradients between the exterior and the interior regions of a material, resulting in plastic deformation of the exterior and elastic deformation of the interior of the material.
Residual stresses are currently measured in a number of different ways. For example, strain gages are used to measure stress just after a specimen is removed from the material by drilling. But drilling imparts its own stresses. Moreover, stresses tend to concentrate at holes including drilled holes. Alternatively, chemical etching can be substituted for drilling, but not all materials can be etched and etching is slow. Furthermore, both etching and drilling are destructive methods of measuring stress.
There are of course nondestructive stress measuring techniques. For example, in the case of untextured, single-phase materials under uniaxial loads, acoustic techniques have proved successful. For magnetic alloys, where the stresses do not exceed 70,000 pounds per square inch, magnetic techniques are applied. If the material is crystalline, X-ray diffraction techniques are often used. But the equipment needed is cumbersome and the depth of the stresses measurable with X-ray diffraction is very shallow, not more than approximately 20 micrometers in most metals.
Optical methods for measuring strain are known; see Chiang's description in U.S. Pat. No. 4,722,600. Laser interferometry is also known in measuring deformation. Kobayashi et al describe interferometry using helium-neon lasers in their patented optical deformation measuring apparatus, in U.S. Pat. No. 5,166,742. In their specification, they disclose that deformation, in particular "movement," of an object can be analyzed by comparison of speckle patterns produced by the object before and after movement. The analysis of speckle patterns is accomplished by Fourier transforms of the patterns.
Using lasers to anneal a small portion of a surface in the measurement of residual stress is also known. Both Viertl et al, in U.S. Pat. No. 4,249,423, and Thompson et al, in U.S. Pat. No. 4,248,094, disclose a method for measuring stress dynamically using strain gages in combination with heat deposition by lasers. The heat causes local melting to produce stress relief.
Hung et al (U.S. Pat. No. 4,139,302) disclose a method and apparatus for analyzing stress-induced deformations using interferometry. Their method involves preparation of an interferogram using coherent light directed toward an object to which stress has been applied, and then processing the interferogram through a fringe-frequency discriminatory filter to make the fringes visible to the eye.
However, nothing in these patents or methods teaches a method for measuring residual stress in an object by the combination of laser interferometry and laser annealing.